In lithographic printing of color photographs, normally a set of four color-separation images of the photograph is made, one for each of the three colors used in printing, namely, cyan, magenta and yellow and one for black. These are dot pattern transparencies which are made by exposing through a screen and appropriate color filters. Then corresponding lithographic plates are made from each of the separation images and each image is printed on paper or other stock by successive, registered contact with the plates, each of which is inked with the color for the particular separation. If the separation images are accurate and if other conditions are satisfied, a good color print is obtained. If, however, one or more of the separation images is not accurate, for example, through even a minor error in development, the color print will be out of balance. If the error is not discovered before the plates are made or before printing begins much waste will occur. Consequently, color proofing before the printing run begins is essential.
Color proofing has been done in the past by conventional silver halide photography, requiring time-consuming chemical processing of the prints. An improved way of color proofing used electrophotographic techniques. This reduces the time for printing and developing each color separation image and for obtaining the composite proof. An especially useful electrophotographic technique involves forming the color proof image with liquid toners on a thin transparent plastic sheet and then adhering the side of the sheet on which the toner image is printed to a paper support sheet. The resulting color image is covered by the thin protective transparent plastic sheet.
One of the qualities of lithographic press prints that a color print should reproduce or simulate is the quality of dot gain. This occurs in a press run because of the fact that when a half-tone lithographic plate applies liquid ink to a paper sheet or other substrate, the inked dots spread slightly because the liquid ink spreads out when the plate presses against the paper.
When a color image is formed electrophotographically on a transparent plastic sheet and is then applied to paper stock the dots do not spread on the paper as do printing inks because the toner is already dry when the plastic sheet is fixed to the paper. There will, therefore, be a slight but undesirable difference between the appearance of the electrophotographic color proof and the press print. The present invention provides an improvement in the electrophotographic color proofing method to simulate the dot spread or "dot gain" which occurs in press runs.